Electromagnetic induction has been used to heat soldering irons. An advantage of using induction heating is the reduced heat loss of the working tip as compared to indirect methods of heating in which heat from a separate resistance coil heater must be transferred to the working tip. Electrical eddy currents are induced in the working tip of the soldering iron. The eddy currents are generally concentrated at or near the surface of the working tip. Due to electrical resistance, eddy currents lead to Joule heating of the working tip. In cases where the working tip is made of a ferrous material, additional heating occurs through magnetic loss or hysteresis heating, which arises from the rapid flipping of magnetic domains inside the material.
As the temperature of the working tip increases, the eddy currents are induced not only at the surface, but also at regions deeper inside the working tip. Above a certain temperature, the eddy currents on either side of the working tip begin to cancel each other resulting in the loss of induced current in the working tip, which leads to a decrease or cessation of Joule heating. Also, resistance to current flows and Joule heating decreases as temperature increases. In addition, ferrous materials loose their magnetic properties at the Curie point or temperature (Tc), which means they do not exhibit hysteresis heating above Tc. The Curie point is a property of the material of the working tip. For example, iron has a Tc of about 1043 K or 770 degrees C. As the working tip drops below Tc, Joule heating increases and hysteresis heating resumes. In this way, the temperature of the working tip may be autoregulated at Tc without the use of a temperature sensor.
One disadvantage of relying solely on Curie point autoregulation is that the working tip is maintained only at a single temperature. It is often desirable to keep the working tip of a soldering iron below a particular temperature to avoid overheating delicate electronic components, such as integrated circuits. However, the ideal soldering temperature may vary from component to component. Also, it may be desirable at times to have a higher working tip temperature to solder larger or less sensitive components more efficiently. Since Tc is a function of the working tip material, different working tips would have to be used for different soldering tasks, which can be inconvenient and costly. Another disadvantage of relying solely on Curie point autoregulation is that inherent manufacturing variances in the working tip material and variances during assembly would cause one working tip to have a slightly different temperature than another working tip, which may lead to inconsistent quality in the work product.
Accordingly, there is a need for a system and method for induction heating of a soldering iron that can be precisely adapted for various soldering tasks in a convenient and cost-effective manner.